1. Field of Invention
This invention relates to cathode ray tube electron guns. More particularly, the invention relates to an electron gun configuration and a method for improving the electron beam landing geometry at the extreme edges of a cathode ray tube viewing screen.
2. Related Art
Cathode ray tubes (CRTs) used in consumer electronics, e.g., television receivers, must present good picture quality. One desirable quality is uniform picture brightness and color purity over the entire viewing screen. That is, a uniformly bright white picture should result when the CRT electron gun excites all viewing screen phosphor elements to emit visible light. Another desirable quality is good focus for the displayed picture. Both qualities depend on proper landing geometry of the electron beam incident on the excited phosphor. Proper landing geometry is difficult to obtain, especially in the corners, with viewing screens that are nearly flat and that have a high width to height aspect ratio such as 16:9.
Picture uniformity requires that the beam width of the electron beam portion striking the phosphor elements be uniform over the entire phosphor area. For example, FIG. 1 is a simplified representational plan view showing a cross section of a typical SONY(copyright) TRINITRON(copyright) CRT, such as a model 36RV, and electron beams directed to excite phosphor stripes that emit colored light. As shown, composite electron beam 20 originates from three electron sources (e.g., cathodes) 22, 24, and 26. Persons skilled in the art will understand that each source 22, 24, and 26 is controlled by circuits that decode a television picture signal, each source emitting electrons so as to energize colored light emitting phosphors to create a color picture. Thus electron beam 20 may include component beam 28 that energizes phosphors emitting blue light, component beam 30 that energizes phosphors emitting green light, and component beam 32 that energizes phosphors emitting red light.
Beam 20 is directed against aperture grill 34 in which aperture slits 36 are defined. In this example, two slits 36 are shown. Portions 28a and 28b of beam 28 pass through the aperture slits 36 to illuminate, for example, blue phosphor stripes 38. Similarly, portions 30a and 30b of beam 30 illuminate, for example, green phosphor stripes 40, and portions 32a and 32b of beam 32 illuminate, for example, red phosphor stripes 42. As shown, phosphor stripes are separated by carbon stripes 44.
The cross-sectional area of beam 20 incident on phosphor screen 35 is the spot size. The cross-sectional shape of beam 20 incident on phosphor screen 35 is the spot shape. As discussed below, spot size and shape are important to achieving proper focus.
The width of the electron beam portions incident on the phosphor stripes is the beam width. Beam width is a critical factor in controlling the landing performance of an electron beam portion incident on a phosphor stripe. FIG. 2 is a simplified cross-sectional view of an electron beam portion, e.g., portion 30a, passing though aperture slit 36 and incident on a phosphor stripe, e.g. stripe 40. As shown, the beam width is somewhat wider than the width of aperture 36 due to scattering effects persons skilled in CRT design will understand. Persons skilled in CRT design will also understand factors that effect landing performance, such as the change in gaussian energy distribution over the beam width and the diffraction occurring as the beam passes through an aperture. For good landing performance, portion 30a is aligned so that the beam width uniformly overlaps carbon stripes 44 on either side of phosphor stripe 40, shown as position 46. Uniform phosphor stripe coverage ensures uniform energy distribution to excite the phosphor stripe for maximum brightness. It can be seen that if portion 30a is shifted to the left or right, for example to position 48, landing performance may decrease. Similarly, if beam width is too wide or too narrow, landing performance decreases because the energy of the electron beam portion is not optimally distributed over the phosphor stripe. Accordingly, there is an optimum beam width and position for an electron beam portion incident on a phosphor stripe.
To ensure picture uniformity, landing performance must be the same for every beam portion incident on every phosphor stripe over the entire viewing area. Persons skilled in CRT design will understand that without any correction, landing performance in the center of the CRT viewing area differs from performance at each of the corners due to the increased deflection of the electron beam and the increased distance from gun to screen. But in addition to landing performance, good focus must be maintained over the viewing area as well. Focus performance is primarily based on spot size and shape.
Factors such as the earth""s magnetic field distort spot size and shape as the beam is scanned over the aperture grill. The most severe distortions typically occur in the corners of the viewing screen. Furthermore, since the CRT viewing area is typically rectangular, horizontal and vertical spot size and shape distortions (beam cross-sectional astigmatisms) differ at the corners due to the length of the respective deflections. Persons skilled in CRT design will be familiar with various conventional correction methods such as SONY""s Auto Beam Landing Correction (BLC), Multi-Astigmatism Lens System (MALS), and Extended Field Elliptical Aperture Lens (EFEAL).
To achieve good focus, the beam cross-section is shaped to ensure proper spot size and shape over the entire viewing screen. Since the spot size and shape changes as the beam is scanned across the screen, the shaping must be dynamic so as to vary with beam position. In TRINITRON(copyright) systems, the beam is shaped using an electromagnet positioned around the main focusing grid in the electron gun, as discussed below.
FIG. 3 illustrates electron gun 49 and beam shaping and deflection components used in a typical TRINITRON(copyright) CRT. As shown, three cathodes 50a, 50b, and 50c, produce electrons in response to signals from conventional circuits (not shown) that decode a color television picture signal. Electrons are directed as shown through a series of grids G1, G2, G3, G4, and G5 to produce a composite electron beam that excites colored light emitting phosphors as described above. Grid G4 is the main focusing grid, and in some electron guns component beams 54a, 54b, and 54c converge in grid G4. Conventional focusing is performed in grid G4 using focusing elements (omitted for clarity) driven by focus voltage driver 51 that supplies focus voltage VF on lines 53 to terminal 53a on grid G4. Beam 54 is focused to produce good spot size as beam 54 sweeps across aperture grill 55 to illuminate phosphor coating 64 on viewing screen 66. Persons skilled in CRT design will understand the details of beam focusing.
Persons skilled in CRT design will also understand the use of a four-pole electromagnet to alter beam spot shape. (See, e.g., U.S. Pat. No. 3,946,266, assigned to the present assignee and incorporated herein by reference.) The following brief discussion illustrates basic concepts. Electromagnet 52 with four poles is positioned around grid G4. As depicted in FIG. 3, only the top two poles 52a and 52b are shown. As described herein, the electromagnet is referred to as Dynamic Quadra-Pole (DQP) magnet. FIG. 4 is a representational side view of DQP magnet 52 with poles 52a, 52b, 52c, and 52d positioned around grid G4 (omitted for clarity). As shown, electron beam 54 travels out of the paper towards the viewer. DQP driver 56 is connected to DQP magnet 52 using lines 58. DQP driver 56 controls the magnetic fields among poles 52a-52d, represented by field lines 60, by supplying DQP current iDQP along lines 58. Thus current iDQP varies as a function of beam position. Persons skilled in CRT design will understand that the spot size and shape of beam 54 may be shaped by varying iDQP to move the magnetic fields through which beam 54 travels. In practice the required iDQP is first simulated, and then fine tuned for an actual sample. The DQP is effective for TRINITRON(copyright) CRTs because of the single beam convergence point in grid G4.
Referring again to FIG. 3, spot size and shape are also influenced by directing each of the three component electron beams 54a, 54b, and 54c through three corresponding shaped apertures in each of grids G1 and G5. Thus grid G1 has three unique apertures, one for each component electron beam 54a, 54b, and 54c. After these component beams converge in grid G4, composite beam 54 is directed through a single aperture in grid G5. The apertures have a small deviation (or xe2x80x9castigmatismxe2x80x9d) from circular. Current CRTs have apertures in which the height:width (vertical:horizontal) aspect ratio is approximately 98:100 (98 percent astigmatism). This 98 percent astigmatism, combined with the changing DQP magnetic fields and the focus voltage, helps to correct the spot size and shape so as to improve landing performance at the edges of phosphor coating 64 at viewing screen 66 in CRT envelope 68 (partially omitted for clarity). Prior to the present invention, CRT engineers believed that an aperture astigmatism and DQP magnetism are fully supplementary. Therefore 98 percent was selected to reduce DQP circuit power consumption.
Beam deflection for scanning is typically carried out by conventional deflector electromagnets (deflection yoke), represented by electromagnets 70 and 72. Persons skilled in CRT design are familiar with various beam deflection methods using electromagnets. Note that for the corners of phosphor coating 64, the horizontal beam 54 deflection is greater than the vertical beam 54 deflection. Accordingly, even though focus voltage and iDQP change, the spot shape tends to be distorted wider horizontally than vertically. If the minimum spot size requirement is ignored, however, a circular spot shape can be obtained with correct focus voltage and DQP current.
The focus voltage not only controls spot size and shape, but also affects the beam width (FIG. 2). FIG. 5 is a graph plotting beam width against focus voltage. As predicted by simulation and verified by measurement, in a conventional CRT, such as described in relation to FIGS. 3 and 4, the focus voltage required for the optimum xe2x80x9cjust in focus pointxe2x80x9d is not the same as the focus voltage required for minimum beam width. For example, for the curve shown, a minimum beam width occurs at the focus voltage VMBW for point A, but the just focus point occurs at the focus voltage VF for point B. (The other minimum beam width point indicated at the lower focus voltage is not considered because it produces an unacceptably large spot size.) The actual beam width changes as the focus voltage varies during normal operation. What is desired is to simultaneously optimize both the beam width incident on the phosphor elements that is required for picture uniformity and the spot size and shape required for good focus.
In accordance with the invention, the electron beam in a CRT electron gun is shaped by passing through one or more apertures having an astigmatism. If one aperture is given the astigmatism, the aperture is given a 90 percent astigmatism. In one embodiment only the aperture in grid G5 is given the 90 percent astigmatism, and current in the DQP magnet is made sufficient such that minimum beam width occurs closer to the just focus point voltage. In another embodiment, astigmatisms in the apertures in both grids G1 and G5 combine to produce an effective 90 percent astigmatism. For example, apertures in grids G1 and G5 are each given a 0.95 astigmatism, thereby producing approximately a 90 percent total astigmatism (0.95*0.95=0.9025). In some embodiments in which astigmatisms in the apertures in both grids G1 and G5 produce the effective 90 percent astigmatism, DQP current is not used to further shape the electron beam. Combined G1 and G5 astigmatism embodiments without DQP correction offer a cost saving solution. In other embodiments in which the apertures in both grids G1 and G5 produce the effective 90 percent astigmatism, DQP current may be used to further shape the beam and produce a better result. Changing the aperture astigmatism, and further using proper current through the electromagnet, allows the focus voltage at which the just focus point occurs and the focus voltage at which minimum beam width occurs to be much closer together. Accordingly, the picture becomes more uniform over the entire viewing area, especially in the corners.